A workpiece or work that is exposed to light such as a printed circuit board is subjected to printing operation together with a mask containing a predetermined pattern so that the pattern may be printed on the work by a printing apparatus. In general, the pattern of one mask is projected onto a number of works continuously. Accordingly, works and masks have been entirely separately treated.
More specifically, in a conventional printing apparatus, an exposure device mounted in the center of the apparatus is open and has transparent plates. First, a desired mask is manually installed in a certain positional relation to the transparent plates and held in position. Then, a desired number of works are fed into the printing apparatus, and the works are separately and successively installed in a given positional relation to the transparent plates. Subsequently, the works are exposed to light and carried away from the printing apparatus, thus obtaining the desired number of printed works.
The conventional printing operation as described above presents no serious problems when a large number of works are consecutively printed using a single mask. However, when a limited number, say 20, of works are printed using one (two in case of double-sided printing) mask, it is quite cumbersome to install masks relative to the transparent plates and to remove them. In particular, since the transparent plates are located at the center of the exposure device constituting a key component of the printing apparatus, it is cumbersome to open the exposure device for allowing the transparent plates to be taken out of the printing apparatus. Further, when a mask is being installed, the transparent plates are taken out of the apparatus, and therefore dust tends to adhere to both the exposed transparent plates and the mask, thereby increasing the possibility of occurrence of substandard products. In addition, because the transparent plates are situated in the center of the exposure device, and because components are disposed and driven around the transparent plates, various mechanisms are placed around the transparent plates, making it difficult to obtain a sufficient space. This renders manual installation and removal of masks quite difficult. Each mask is marked with many symbols on its peripheral portion to indicate its kind. If a finger tip makes contact with this peripheral portion and soils it, then the symbols may not be read. To prevent this undesirable situation, the peripheral portion of the mask is provided with a portion of a given width that is grasped while the mask is being handled. The symbols are put down on the inner side of the grasped portion. Hence, the effective area available for marking of a pattern is small as compared as the size of the mask.
In an attempt to eliminate the foregoing difficulties with the conventional printing operation, a new printing apparatus has recently been proposed. According to this proposed apparatus, a storage device in which two kinds of pallets are mounted in stacked and vertically movable relation to each other is mounted near the apparatus. Each of one kind of the pallets receives a desired number of individual masks, while each of the other kind receives a desired number of works. First, one pallet in which a mask to be used is housed is moved upward or downward to a given position. The pallet is then advanced into the printing apparatus and halted in a certain positional relation to transparent plates. Then, a mask is pulled up from inside the pallet by means of a conveying device in the printing apparatus. During this operation, the pallet is moved back into the storage device. Then, the conveying device is lowered to mount the mask in a given positional relation to the transparent plates. Thereafter, another pallet that receives works is advanced from inside the storage device to a given position relative to the transparent plates, and this pallet is mounted in this position in the same way as the mask. Finally, the works are exposed to light. It is certain that this proposed apparatus permits mounting and dismounting of masks relative to the transparent plates to be automatically done and that it is able to eliminate various difficulties associated with the handling of masks. However, this apparatus needs the special-purpose storage device and a computer for controlling the operation of the storage device in accordance with a program, increasing the cost of equipment. Additionally, a larger space is required to operate the printing apparatus. Further undesirably, whenever each work is mounted in a certain positional relation to the transparent plates, a pallet holding works therein must be moved back and forth between the storage device and the transparent plates. Therefore, it takes a long time to expose each one work to light, leading to deterioration in the efficiency with which the operation is performed.
Also, because each pallet usually disposed outside the printing apparatus is brought to a position above the transparent plates in the exposure device lying in the center of the apparatus, it is highly possible that dust or the like adhering to the pallet is conveyed above the transparent plates, and it has not been possible to greatly reduce the occurrence of substandard products due to adhesion of dust.